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Full text of "Hacks and Mods"

One-Ton Linear Servo 



i 



Make Projects 



build, hack, tweak, share, discover, J 



One-Ton Linear Servo 



Written By: Windell Oskay 



f TOOLS: 

• Desoldering Braid (1) 

• Multimeter (1) 

• Power source (1) 

such as a car battery, car cigarette 
lighter output, or equivalent 

• Screwdrivers (1) 

• Soldering iron and solder (1) 



© PARTS: 



Scissor jack (1) 

$60 at major auto parts stores 

Servo (1) 
any type 

Servo controller (1) 
or R/C receiver 

Potentiometer (1) 

such as Digi-Key part #3590S-2-502L- 

ND, http://digikey.com 

Relays (2) 

Hamlin HE3621A0510 (or similar). Digi- 
Key #HE207-ND 

Transistors (2) 

Resitstors (2) 

Capacitors (2) 

Trimpots (2) 

such as Digi-Key #3362M-501LF-ND 

LEDs (2) 

resistors (2) 

Hookup wire (1) 



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Page 1 of 7 



One-Ton Linear Servo 



Protoboard (1) 

such as BG Micro #ACS1053 or 

RadioShack #276- 149 

Masking tape (1) 

String (1) 

Washer (1) 

Wood(1) 

for mounting setup 

Spring or big rubber band (1) 

Weight (1) 

e.g., a can of soup 

Screw eyes (2) 



SUMMARY 

RC hobby servomotors were just made for hacking. What's not obvious is how much you 
can hack them — with a few tricks, you can use a servo to control almost anything. 

Hobby servos consist of a DC gear motor, a potentiometer (usually 5K), and a control 
circuit. The output shaft is the output of the geared motor. The shaft also turns the pot, which 
returns a position-dependent voltage to the control circuit. When the servo receives a 
command to move to a new position, it runs its motor in the necessary direction until the pot 
indicates that the output shaft has reached the desired location. 

What if you want motion more powerful or complex than a standard servo can give — can 
you do it? Certainly! Just replace the little motor with a bigger one (and whatever driver is 
needed) and/or replace the pot with one that senses the movement that you really want. 

Let's get started with extreme servo hacking. At Evil Mad Scientist Labs , we've modified a 
standard hobby servo to control an automotive jack. The result is a powerful, sub-$100 
actuator that can move a heavy load with precise control, with a total stroke of 5"-10". It's a 
powerful weapon in the maker's arsenal for theatrical and Halloween props, CNC projects, 
and robotics applications. 



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Page 2 of 7 



One-Ton Linear Servo 







We'll start with a 1-ton, 12V automotive scissor jack. The controller has buttons labeled 
"up" and "down." Open the controller to find the backside of the up button. With your 
continuity tester, find the 2 pins of the button that become connected when you press it. 




Solder 2 long wires to those pins. Repeat for the down button, adding 2 more long wires 
across it. 

Reassemble the controller. You may need to make a little notch for the wires to escape. 



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One-Ton Linear Servo 





• Next, you need to eviscerate a 
servo. The best candidate is one 
with a bad motor, a broken case, 
stripped gears, or a bad pot. 
Unscrew and open the case to get 
at the printed circuit board (PCB). 
The appearance may vary 
tremendously, but the features 
shown in 



inear_servo_diagrams.pdf (in the 
documents section) are universal: 
an input (signal and power) cable, 2 
outputs that go to the motor, and 3 
wires that go to the pot. 

Cut these latter 5 wires in half, 
carefully noting which 2 went to the 
motor and which one went to the 
middle (wiper) terminal of the pot. 
(Some servos have the motor 
and/or pot soldered directly to the 
PCB without wires. In that case, 
unsolder and remove the part(s) 
instead of cutting the wires.) 

You're replacing the servo's 
internal single-turn pot with an 
external 10-turn pot. Solder 3 long 
wires — with a different color for 
the wiper — from the original pot's 
contact tabs to the 10-turn pot, 
connecting wiper to wiper. (The 
wiper pin of a 10-turn pot is usually 
at the end opposite the shaft.) 



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One-Ton Linear Servo 





• To control the jack, you'll use little 
relays driven by adjustable, single- 
transistor driver circuits. Begin by 
identifying which of the 3 wires on 
the servo's control cable are power 
(+5V) and ground, and solder 
connections to those points. 

• Construct output driver #1 as 
shown in the linear servo pdf, 
watching the orientation of the 
capacitor (its negative side is 
marked), the transistor (its legs 
read E-B-C when you can read the 
writing), and the relay (its pin 1 is 
marked). Take either wire that 
previously went to the motor as the 
driver's input, and connect the 
driver's output to the 2 wires 
across the up button on the jack. 
An optional LED can be added for 
debugging. 

• Make an identical output driver #2 
with the other motor wire, and 
connect its output across the down 
button. 



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One-Ton Linear Servo 





• The jack's movement must turn the 
pot in order for servo control to 
take place. Two working 
geometries are sketched in 
geometries.pdf in the documents 
section. 

• You can tie a string to the top of 
the jack, wrap the string one turn 
around the shaft of the pot, and 
then hang a weight below to 
maintain tension. 

• Or, a more general scheme is to 
wrap the string through a screw 
eye, around the pot, and then 
attach the other end through a 
rubber band (or spring) to a second 
screw eye. 

• For either method, the pot's shaft 
needs some friction: wrap regular 
masking tape around the shaft 10 
times, and then slide a W-ID 
washer over the tape. This holds 
the tape in place and stops the 
string from rolling off the edge of 
the tape. 



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One-Ton Linear Servo 





• Center both 500Q trimpots in their 
ranges, and hook up your 12V 
source for the jack. Manually 
adjust the jack to middle height, 
move the 10-turn pot to mid-range, 
and then tension the string. 

• Turn on your servo control signal 
and see if it works. The jack's 
movement should "lock up" 
wherever you want it to stop. If it 
doesn't lock up — the jack moves 
to one end of its travel regardless 
of signal — then the feedback is 
backward. Fix it by swapping the 2 
non-wiper wires going to your pot. 

• The 2 trimpots adjust the "dead 
band" of the servo; tune it between 
a loose lock, where precision is 
lower, and a too-tight lock, which 
may oscillate. 



This project first appeared in MAKE Volume 18 , page 123. 



This document was last generated on 201 2-1 0-31 02:27:36 AM. 



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Page 7 of 7